1. Field of the Invention
The present invention relates to an information recording/reproducing apparatus using an electrical phenomenon caused by applying a voltage across a probe and a sample (recording medium) and bringing the probe and the sample close to each other and, more particularly, to a control system for controlling a current value flowing through the probe and the sample in recording.
2. Related Background Art
In recent years, a scanning tunnel microscope (to be referred to as an STM hereinafter) which allows the operator to directly observe the surface of a substance and an electronic structure near the surface by bringing a probe and a sample close to each other and using a physical phenomenon (tunnel phenomenon or the like) caused at that time has been developed [G. Binning et al., Helvetica Physica Acta, 55, 726 (1982)]. This STM enables measurement of a real spatial image at a high resolution regardless of a single-crystal or amorphous medium. The STM also realizes observation of a medium at a low power without damaging the medium by a current. The STM operates in not only ultra-high vacuum but also the atmosphere and a solution, and can be employed for various materials. The STM is expected to be widely applied in the academic field or the field of studies.
Also in the field of industries, the STM has recently received a great deal of attention because of its principle that the STM has a spatial resolution in atomic or molecular size. As disclosed in Japanese Laid-Open Patent Application Nos. 63-161552 and 63-161553, applications and practical use of the STM to an information recording/reproducing apparatus by using a recording layer in a medium are extensively being progressed.
In this information recording/reproducing apparatus, information is recorded by moving the probe to a designated point on the medium, applying a voltage (bias), displacing the recording medium from a high-resistance state to a low-resistance state, and writing a spot on the recording medium. For example, the low-resistance state is set as a bit ON state to allow to write binary information on the recording medium.
However, even after the recording medium is switched to a low-resistance state, an excess current flows for a long time because a bias is kept applied. As a result, Joule heat may be generated to destroy the film of the recording medium or the tip of the probe in some case.
The flow (flowing time and amount) of the current changes, so the bit size varies.